The mobile broadband business is growing rapidly. User devices are becoming cheaper and data rates are reaching higher. New services and applications that could not be supported previously are becoming regular features. Those new services and applications may require constant or frequent access to a mobile network, for example, even when the user is in a vehicle such as a car, truck, train, or aircraft. Most often the vehicle is moving fast and is enclosed by metallic surfaces, which poses difficulties for a mobile device inside the vehicle communicating with a nearby mobile network. An enclosed vehicle resembles a Faraday cage and attenuates the radio transmissions to and from the vehicle due to the well-known Faraday cage effect. Besides vehicles, buildings having walls made of metallic materials or rooms partitioned out using metallic materials also make radio communication between an inside mobile device and an outside mobile network difficult.
Both repeaters and relays have been used to improve radio link quality and to increase coverage of a mobile network. The general idea of relaying is that a relay node (or simply relay) receives a transmission from a sender and forwards the transmission to the intended receiver. For example, the sender may be a base station or an evolved NodeB (eNB) and the intended receiver may be a mobile device or user equipment (UE), or vice versa. A repeater functions similarly as a relay. But there are some differences between the two. One of the differences is that a relay often decodes and reconstructs a transmission before forwarding while a repeater merely amplifies a received transmission before forwarding. A repeater generally receives, amplifies, and forwards a transmission without any link layer interactions, for instance, decoding. A repeater is sometimes referred to as a radio-frequency (RF) relay, or layer 1 (L1) relay. Term “L2 relay” or “L3 relay” are also used. The different layers refer to the layer of the protocol stack where the transmission is received, either by the relay or by the intended receiver.
It is known that a repeater or a relay node can be used in an enclosure, e.g., a building or a cage, to improve radio communication. A pick-up antenna outside the building or cage receives a radio signal and feeds the received radio signal to the repeater or relay. The repeater or relay may be located inside the enclosure. A second antenna inside the cage is used to transmit radio signals from the repeater or relay node to the intended mobile device or UE located inside the enclosure. Ideally, the transmissions between the second antenna and the mobile device are isolated within the enclosure and will not interfere with outside radio signals. Also the metallic surface of the enclosure attenuates outside radio signals and substantially reduces outside interference on the radio communication inside the enclosure. However, in general, the inside radio transmissions and the outside radio signals can't be perfectly decoupled. One reason is that the enclosure often has openings or apertures, for instance, lowered windows on a vehicle or opened doors of a building.
Nowadays, car windows are often treated with a metallic surface for various reasons. The position of a car window coated with a metallic surface affects both the transmissions of the relay/repeater located inside the car and the transmissions of a radio transmitter located outside the car. When the window is up, the two kinds of transmissions are decoupled to a certain degree. When the window is lowered, the transmissions from the inside relay/repeater interferes with outside radio signals. Also a nearby mobile device located outside the moving car may attempt to connect with the inside relay node/repeater, which may be only in transit. It is a challenging task to ensure that a relay/repeater located in a vehicle works properly in different scenarios and that an inside relay/repeater does not interfere with external radio communications.
There is a need for advanced techniques that can be used to improve radio link performance for a radio transmitter located within an enclosure.